Photochromic ophthalmic lenses made of mineral glass are well known. Photochromic dyes have good compatibility with mineral glass. The most common complaints about photochromic mineral glass lenses concern the weight of the lenses and the slow photochromic reaction time, particularly the change from dark lenses to light lenses.
Unlike the lens industry of years ago when lenses were made only from glass, now spectacle lenses are made from many different types of plastic or from glass-plastic composites. Plastics include acrylic, PPMA (a product of PPG-Pittsburgh Plate Glass) also known as CR-39.RTM., and Lexan.RTM. (a polycarbonate made by General Electric).
Recently, attempts have been made to apply photochromic dyes to light-weight plastic lenses to render them similarly photochromic. However, for various reasons this objective has not been satisfactorily achieved with plastic lenses. One reason for the lack of success has to do with the chemistry of ethylene glycol diallyl dicarbonate, the most commonly used monomer for producing plastic ophthalmic lenses. This monomer is cast in a lens mold and polymerized with a catalyst such as isopropyl percarbonate. One might expect that a plastic lens made from such a monomer could be rendered photochromic simply by incorporating photochromic dyes into the monomer composition prior to casting the lens. However, in practice it was found that, following polymerization of the organic material, the photochromic dyes did not retain their photochromic property. Apparently, the catalyst required for the polymerization caused inhibition of the dyes. Thus, it has not been possible in practice to simply incorporate photochromic dyes into the monomer composition when making ophthalmic lenses from an organic material.
Advances have recently been made on two fronts in materials and methods for the on-site production of prescription lenses: advances designed to reduce the amount of stock materials which must be carried on hand, and advances in reduction of the time needed to produce the prescription lenses. However, none of these approaches has made it possible to produce plastic photochromic prescription lenses. Further, none of the available methods permits the lens crafter to utilize simple stock lenses, yet offer a wide variety of photochromic properties, tinting, UV protection, scratch resistance, and other desirable properties. Further yet, none of the available methods or materials makes possible the provision of photochromic lenses having an extremely rapid photochromic response rate.
A typical approach to on-site custom lens production involves casting an additional plastic layer onto a plastic lens blank. See, for example, in U.S. Pat. No. 2,339,433 (Staehle) disclosing a method of adding a correction to a molded plastic lens by adding a thin level of resin. U.S. Pat. No. 3,248,460 also discloses means for casting plastic lenses from thermosetting or thermoplastic materials wherein a plastic blank having significantly less curvature than required for the full intended prescription of the lens is used as a base. An additional layer of material is cast onto this base. The patent employs a conventional optical gasket to provide space between the plastic blank and the mold and to hold the resin material in the cavity created thereby. The additional layer of material changes the curvature of the resulting lens over the vast majority of its surface, thereby changing the prescription of the resulting finished lens to the power required. The material is cured by heat. Such a heat curing process requires heating over a period of more than 12 hours, thus making the formation of the lens a long, drawn-out process. Obviously, photochromic lenses could not be produced from such materials and methods.
Others have tried to manufacture multifocal or progressive plastic lens using a lamination technique. Such a technique joins a preformed plastic section, referred to as a wafer, to a matching cured plastic prescription lens. The preformed wafer section defining a multifocal or progressive region of the finished lens is joined to the prescription lens by adhesive. The adhesive for bonding the two lens components is the same monomer used to cast the lens components. Accordingly, no provision is made for incorporation of photochromic dyes.
U.S. Pat. No. 5,462,698 (Kobayakawa, et al.) entitled "Photochromic Composition" addresses the problems associated with specific photochromic compounds which tend to be slow-acting or inactive when incorporated in plastic, and solves the problem by use of a resin compound having at least one epoxy group in the molecule as the resin for forming the photochromic lens. Kobayakawa, et al. (a) is directed to forming a lens having photochromic compound disbursed throughout, (b) requires the presence of multiple types of photochromic compounds in combination, (c) requires the use of a polymerizable compound having at least one epoxy group to form the lens, (d) requires polymerization in a heat furnace, with polymerization taking from 2 to 40 hours, and (e) reports fading time to 1/2 density measured after exposure to 60 seconds averaging 3 minutes (Table 1). Kobayakawa, et al. thus requires a long time to produce a specific type of slow acting lens.
More recently, U.S. Pat. No. 5,531,940 (Gupta et al.) teaches methods for making optical plastics lenses with photochromic additives. According to a first embodiment of the invention, a casting resin having a low cross link density comprising polymerizable components (preferably including up to 50 wt % bisallyl carbonate) and photochromic additives, wherein all polymerizable components have a functionality not greater than two, is arranged between a mold and a lens preform and then cured. Upon polymerization the resin has a low cross-link density and forms a soft matrix. This soft matrix is unsuitable as the outer layer for photochromic lenses. According to a second embodiment of the invention, the casting resin but substantially free of photochromic additives is arranged between a mold and a lens preform and then cured. The resin is then impregnated with photochromic additives. In a third embodiment, the layering resin containing a photochromic additive is provided on the surface of a mold and cured to a gel state. Then, a casting resin that is substantially free of photochromic additives is arranged between the coated mold and a lens preform and cured. According to a fourth embodiment, a casting resin that is substantially free of photochromic additives is provided on the surface of a mold and cured to a gel state. Then, a casting resin containing photochromic additives is arranged between the coated mold and a lens preform and cured. There is no discussion of photochromic rate of reversal, and the photochromic material is represented as being too soft to expose to the environment. Further, the process is complex, time consuming, and requires a high degree of attention.
Accordingly, it has not been possible previously to simply coat a lens with a photochromic material and thereby render the lens photochromic. Even if a coating composition had been available for rendering lenses photochromic, the requirement for forming a very even coating has in the past required certain precise coating techniques in the lens manufacturing industry. For example, U.S. Pat. No. 4,267,208 (Ireland) teaches coating of the convex side of an optical lens for blocking purposes by immersing the downwardly facing convex side in a liquid coating material in a can, upwardly withdrawing the lens above the level of the coating material but below the lip of the can, and spinning the lens about its vertically disposed axis to spread the coating material by centrifugal force as a uniformly thin film over the convex side, to create an edge buildup of coating material to form a thickened bead-like peripheral or rim portion at the periphery of the convex side, and to spin-off excess coating material against the inside of the can for reuse. Upon spin-off of excess coating material the lens is withdrawn out of the can whereupon the film on the spinning lens dries sufficiently for handling. To facilitate the lens coating operation, the lens is held on its concave side by a suction cup having the upper end of its vibration damping stem chucked in the drive shaft of a hand-held motor. To further facilitate the lens coating operation, a releasable vacuum source is operatively associated with the suction cup to pick up a lens for immersing and spinning and to release the suction cup from the lens without handling of the lens. Accordingly, complex apparatus and operations are required to form a coating layer on a lens.
U.S. Pat. No. 5,164,228 (Peralta, et. al.) also teaches the coating of a plastic ophthalmic lens with a scratch-resistant coating using a spin coating process.
There is thus a need for a method for applying a photochromic coating onto non-prescription or prescription ophthalmic lenses, which method would enable any optometrist to easily, quickly, and effectively custom produce photochromic plastic lenses, which lenses may also be tinted, UV-protective, and scratch resistant as required by the customer.